Diminished secretion of growth hormone is responsible in part for the decrease of lean body mass, the expansion of adipose-tissue mass, and the thinning of the skin that occur in old age.
The amounts of cortical and trabecular bone mineral mass were measured by means of microdissection and an ashing technique at approximately 2.5 mm intervals along the most distal 12 cm of radii and ulnae from four women aged 21, 43, 63, and 85. The data show that the distributions of mineral mass and percentage of trabecular bone are similar in both bones. At sites in the radius and ulna commonly used in the photon absorptiometric method of bone mineral mass measurement the percentage of trabecular bone varies between 10% and 50%. The percentage of trabecular bone in the most distal 10% of the length of the radius and ulna remains approximately constant with age but the percentage in the segment which lies between 30% and 40% of the length, measured from the styloid process, increases with age.
H d t h Physics V d . 35 (July) pp. 91-101 P a w o n Press M.. 1978. Printed in Orcat Britain 0 Health Physics Society 00 I7-W78/78/070 1 -00! 3Abstract-The ICRP model, Alkaline Earth Metabolism in Adult Man, predicts that 10% of injected radium-224 atoms decay on bone surface, and that 1.5% decay in bone volume. The model was rerun for all possible values of the size of the bone surface compartment and of its rate constant in an effort to determine the reliability of the bone surface prediction. These runs together with data for 45Ca, "'Ba, and 526Ra in rabbits and dogs lead to lo+ 2% (probable error) as our best estimate for the bone surface fraction of injected '=Ra in man. This estimate corresponds to a time of maximum radium concentration at bone surface in man of 18+6hr, and a preference for radium compared to calcium in the size of the bone surface compartment of between 2 and 3. New measurements of the total surfacelvolume ratio of bone from 10 human skeletons give a preliminary value of 50 cm2/cm3. Assuming 100% retention of thoron and its daughters at bone surface, these input values, together with integrations reported by Mays, give the estimate that an intravenous injection of 1 pCi of 224Ra in a 70 kg standard man with 5 kg of bone results in an average dose to the endosteal cells 0-10 p m from bone surfaces of 1.5rad. Considering all sources of error we believe this estimate is probably good to within a factor of 2.By calculatin the ratio of the endosteal doses for equal average doses to bone for both 224Ra and ' $u relative to 226Ra in man, we give values for what we have called the Relative Distribution Factor (RDF). This factor predicts the relative toxicity of alphaemitting radionuclides in man relative to 226Ra on the assumption that the dose to endosteal cells is the causative factor. The value of RDF('"Rala6Ra) is 20, practically independently of whether endosteal dose is averaged 0-5 p m , 0-10 p m , 0-20 p m , or 0-30pm. The RDF(23%/p6Ra) is about 28 for a surface source of plutonium, and O.% for a volume source of plutonium. The reference for these factors is taken to be a volume source of 226Ra. Since endosteal doses calculated by averaging exactly 0-10 p m from bone surface are somewhat arbitrary for alpha emitters-we don't really know exactly how far from bone surfaces the target cells lie-it is reassuring to find that RDFs for alpha emitters relative to '=Ra are almost independent of distance so long as the cell targets lie within 3 0 p m of the bone surface. Therefore, the RDF with p6Ra as the standard of reference provides a more secure description of the spatial differences between the dose distributions of alpha emitters in bone than does endosteal dose itself.The observed RBE of 224Ra relative to 226Ra for the induction of osteosarcomas-the ratio of average bone doses (226Ra/mRa) for equal incidences-is about 6, a factor of 3 less than the RDF of 20. In other words, by present calculations, 224Ra appears to be a 'Work performed under the auspices of the U.S. Atomic Energy Commissi...
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